The invention relates to a process and apparatus for determining the impedance of a discharge in a plasma reactor associated with a tuning box. It applies more particularly to the regulation of the impedance of the discharge or the ionic current in the reactor.
The invention can be used with any engraving or deposit reactor (for example, deposit by pulverization) supplied by a generator of high frequency electromagnetic waves (more simply called a high frequency generator) by means of a tuning box enabling the output impedance of the generator to be adapted to the input impedance of the reactor. The invention applies, for example, to reactors of the reactive ionic engraving type, reactors using a magnetron, those using a triode.
The term high frequency is understood to mean a frequency lower than 10 GHz (radio frequency, hyperfrequency, . . . ).
The impedance of a discharge in an engraving or deposit reactor is a basic characteristic of the reactor. A knowledge of such impedance gives access to the essential characteristics of the reactor in operation, such as the ionic current (or ionic current density) and the energy of the ions. More particularly these characteristics are important when isotropic engravings are performed, which play an essential role in the production of very highly integrated circuits.
The impedance of the discharge is a complex number Z which is equal to the ratio between the sinusoidal electric voltage V existing between the terminals of the reactor during operation and the also sinusoidal intensity I of the electric current then flowing through the reactor.
Three techniques are already known for measuring the impedance of a discharge in a plasma reactor.
A first known technique consists in measuring the voltage V, the intensity I and the phase shift therebetween, while trying to eliminate the numerous harmonic components of V and I. This first known technique requires an appropriate assembly which it is difficult to use with industrial plasma reactors.
A second known technique consists in measuring or calculating the output impedance of the tuning box associated with the reactor.
This second technique is disclosed in the Article b A. J. Van ROOSMALEN entitled: "Plasma parameter estimation from RF impedance", published in Appl. Phys. Lett. 42(5), 1 March 1983, Pp. 416-418, and also in the Article by N. NORSTROM entitled: "Experimental and design information for calculating impedance matching networks for use in RF sputtering and plasma chemistry", published in Pergamon Press Vacuum, Vol. 29, No. 10, 1979, Pp. 341-350.
However, this second known technique has disadvantages: with this technique the impedance of the discharge is not measured on its own, but measurements are also made of the impedance of the link between the reactor and the tuning box and also the off-load impedance of the reactor, so that a precise analysis must be carried out of these impedances, which must be introduced into correctional formulae; moreover, the tuning box is an imperfect impedance adaptor: a proportion of the incident energy therein is lost by Joule effect or by electromagnetic radiation, something in the long run which prevents the evaluation of the stray impedances represented by the impedance of the link between the reactor and the tuning box and the off-load impedance of the reactor.
A third known technique is described in the Article by B. ANDRIES et al., entitled: "Method of electrical characterization of plasma engraving machines with capacitive coupling", published in the reports of the work of the CIPG 87 by Societe Francaise du Vide, Antibes, 1-5 June 1987, Pp. 106-109. This third known technique consists in calculating the impedance of the discharge from measurements of dispersion factors (known as "S parameters") of the tuning box connected to the turned-off pretuned reactor. The reactor is tuned when its input impedance is made equal to the impedance of the reactor supply line, such impedance generally being 50 ohms, so as to cancel out the power reflected towards the generator. These measurements are performed using a vectorial analyzer at the generator frequency, which is usually 13.56 MHz.
This third technique allows the effective calculation of the impedance Z of the discharge, by integrating the effect of all the parasite impedances, and does not necessitate a perfect tuning box--i.e., the absence of energy losses therein. However, this third known technique has disadvantages: it does not enable the impedance of the discharge to be measured in real time--i.e., continuously--when the reactor is operating (for example, during engraving). Moreover, vectorial analyzers are expensive apparatuses, and it is difficult to envisage their being permanently left on industrial reactors.
It is an object of the invention to obviate the aforementioned disadvantages by providing a process and apparatus which enable the impedance of a discharge in a plasma reactor to be determined, can be used with an industrial reactor and enable the impedance of the discharge to be determined by integrating the effect of all the parasite impedances, without assuming the absence of losses of energy in the tuning box, which do not require the immobilization of a vectorial analyzer on the reactor and, more particularly, enable the impedance Z of the discharge to be determined in real time, without disturbing the plasma.
However, it may be useful to permanently know the impedance of the discharge: in the case of the engraving of an integrated circuit, for example, the impedance varies at the end of engraving and its knowledge may enable the end of the attack of the integrated circuit to be detected; it is also known that reactors become contaminated in the course of time and that such contamination is shown by a variation in impedance, whence the advantage of knowing the impedance of the discharge, to detect such contamination; lastly, more particularly in the case of the engraving of a semiconductor wafer, it is known that the impedance of the discharge can be mathematically connected with the ionic current and with the energy of the ions in the reactor, and the permanent knowledge of the impedance of the discharge allows control of any drift in the course of time of the two parameters formed by this current and this energy, parameters which are essential for engraving.